A. Field of the Invention
The present invention relates to a pushbutton switch which utilizes a buckling plate spring, or more particularly, to an improvement of a pushbutton switch for obtaining electrical contact by reversing the curvature of the dome-shaped buckling plate spring.
B. Description of the Prior Art
In vibratory vehicles, ships, or electric products for domestic use, a pushbutton switch is utilized in which a buckling plate spring is provided because of contact ability as well as human engineering. The normal upwardly concave curvature of the buckling plate spring is reversed to a lower concave curvature upon application of preselected load by the fingertips of an operator, and the displacement of the buckling plate spring advances if the load is loosened, thereby obtaining an electrical communication.
In the prior art pushbutton switch U.S. Pat. No. 3,725,907, as shown in a plan view partly cut away of FIG. 1 or in a vertical section of FIG. 2, a common conductive member 2 and an individual fixed contact 3 are provided on a printed circuit board 1. A conductive dome-shaped click spring 5 whose central portion 4 is curved upwardly is inserted in an insulating spacer 7 which has a retaining hole 6. The peripheral or circumferential edge 8 of the click spring 5 is mounted on the above-said rectangular common conductive member 2, and a dust sheet 9 for environmental seal is overlaid on a central portion 4. A pushbutton 12 which has a flange 10 and a protruding portion 11 is carried within a cover 13 which is put on the dust sheet 9.
The click spring 5 is constructed of a buckling plate spring which, as shown in a load to displacement diagram of FIG. 3, increases its displacement to a certain extent (b) even if a load by a fingertip of an operator to the pushbutton 12 is slackened once the load exceeds a limit (a), while automatically returning to its original state by its own restorative force. The protruding portion 12 causes, via the dust sheet 9, the central portion 4 of the click spring 5 to reverse its curvature through depression of the pushbutton 12 by the operator so that an electrical contact or communication between the common conductive member 2 and the fixed contact 3 is obtained. When communicated, a repulsive force of the click spring 5 to the fingertip of the operator is abruptly decreased (hereinafter called "click effect"), therefore, the switching action can be acknowledged. Once the operator releases the pushbutton 12, the click spring 5 automatically restores to its original state in order to break the electrical communication between the common conductive member 2 and the fixed contact 3.
But, there are many defects in the pushbutton switch as described hereinunder.
First, the contact function tends to degradate or a contact fault to occur. Namely, on every switching action, the circumferential edge 8 of the metal click spring 5 slides on the metal common conductive member 2 which is provided on the print circuit board 1. Accordingly, the sliding portion of each is abraded to cause the degradation of the contact function, moreover, contact fault is caused by the metal filings which are produced through the above-said abrasion. Said contact fault is conspicuous when the common conductive member 2 is printed on the printed circuit board 1 as a circuit pattern.
Secondly, the cost of the production is expensive and a high degree of precision as well as numerous man-hours are required in manufacturing. Namely, the large number of parts increase the manufacturing cost, because the common conductive member 2 and the fixed contact 3 are manufactured as individual parts. Besides, in arranging both of them on the printed circuit board 1, it is necessary to bring both of them into close contact with the printed circuit board 1 so as to equalize the necessary force for depressing each pushbutton switch or corresponding displacement thereby for the before-mentioned click effect. Therefore, a high degree of precision and large number of man-hour are required, especially in manufacturing a key board in which many pushbuttons are provided.
Thirdly, the indicator of the switching action is uncertain. Namely, in the above-said pushbutton switch, the dust sheet 9, which is generally made of a plastic sheet and has a elasticity, is provided between the cover 13 and the spacer 7 in order to prevent the dust which intrudes from a space between the pushbutton 12 and the cover 13. The dust sheet 9 cushions the depressing force to the click spring 5 by the pushbutton 12, thereby the click effect which is characteristic of the pushbutton switch is offset. The chain line of FIG. 3 indicates the offset of the click effect and before-mentioned (a) shifts to (a'). In general, a stroke of the pushbutton 12 in a switching action depends on a throttle depth of the dome-shaped click spring 5 or an allowable stress of an employed material and is originally very short, compared with a cantilever switch. Therefore, the operator is uncertain of the indicator of the switching action.
Fourthly, the necessary force for depressing the pushbutton 12 and the click effect are of variable character. Namely, abovesaid retaining construction in which merely a part of the circumferential edge 8 of the click spring 5 is mounted on the common conductive member 2 often causes the variation because the click spring 5 has a specific roll direction of a spring material.